In electron microscope investigations it is often necessary to be able to displace the specimen not only in the so-called sharp-focusing plane or (shorter) focusing plane, but also to be able to tilt the same about an axis arranged normal to the optical axis of the electron microscope. Specimen holders suitable for this purpose are generally called goniometric plates, or goniometer stages. When such devices are used, it is naturally advantageous if both the section of the specimen and the focusing are maintained when the specimen is tilted. To attain this, the axis about which tilting is performed must intersect the optical axis of the electron microscope in the focusing plane. Such an axis is known as a eucentric axis. Points on a specimen located on this axis remain unchanged in their spatial position when the specimen is tilted; that is, if the eucentric axis is correctly adjusted and thus the desired section of the specimen, for focusing, is brought to the point of intersection between the optical axis and the eucentric axis by suitable positioning devices of the goniometer stage. Then if the specimen is tilted about the eucentric axis the focusing and the section of the specimen are maintained or are dependent only on the accuracy of the mechanical apparatus involved, as long as the positioning devices are rotated in common with the tilting device.
Philips Technische Rundschau, Volume 29, page 378, 1968 discloses a goniometer stage in which the specimen holder rod is seated in a bearing sleeve that is spherically shaped on its end facing toward the specimen. This end of the bearing sleeve is movable in an inner bearing which is fixedly connected to the objective lens block of the electron microscope. The other end of the bearing sleeve is supported, via two positioning devices for two mutually perpendicular directions (Y, Z) in a rotatable cylinder, which is connected to the microscope column via an outer bearing and a tube. When the cylinder is rotated, the bearing sleeve is rotated as well, via the positioning devices, thereby tilting the specimen holder rod. The tilting axis is determined by the axis of the cylinder, and this axis is aligned to the center point of the ball-like inner bearing. The movement of the specimen in the third spatial coordinate (X) is effected by displacing the specimen holder rod in the bearing sleeve via a micrometer spindle and a pendulum rod on the side of the objective lens housing located opposite the tilting device.
The disadvantage of this goniometer stage is that the tilting axis can be aligned in the horizontal plane only by displacing the entire apparatus with respect to the electron-optical axis of the objective lens. Adjusting the elevation of the tilting axis into the focusing plane of the objective lens is effected by mechanical alignment (that is, adjustment upon assembly) in the objective lens block. Since the micrometer spindle is likewise not adjustable with respect to the tilting axis, the precision of manufacture is decisive for maintaining the X coordinate during tilting. It is most disadvantageous that the rotation of the bearing sleeve is transmitted via the positioning devices for the Y and Z directions, which affects the accuracy of the adjustments and has the effect that at high magnification (greater than 100,000.times.), the positioned section of the specimen disappears from the viewing screen of the electron microscope when tilted by a large angle (greater than 45.degree.).
U.S. Pat. No. 4,405,865 discloses a goniometer stage that also has a ball-like inner bearing in the objective lens block. In this bearing, a tube is moved that on its outer end is rotatably journalled in a ball guide bushing. The ball guide bushing is connected via a diaphragm with a fitting that is displaceable on a planar surface of the microscope column and thereby enables adjustment of the tilting axis in two mutually perpendicular directions (Y, Z). The tube that is rotatable about the tilting axis is connected on its outer end via positioning devices, which are for two mutually perpendicular directions (Y, Z), with a bearing sleeve that has a further connection with the tube in the vicinity of the ball-like inner bearing, via an inner diaphragm, and thus can be pivoted by the positioning devices about the center point of this diaphragm. By means of a further positioning device, the bearing sleeve also can be positioned, because of the inner diaphragm, in the direction (X) of the axis perpendicular to the diaphragm surface, that is, in the direction of the longitudinal axis of the bearing sleeve.
The disadvantage of this goniometer stage is that the positioning devices for the Y and Z directions comprise firmly attached link joints, so that the adjustments reciprocally affect one another. Furthermore, the center point of the diaphragm is not exactly defined. Because of the great length of the transmission route, positioning for the X direction is very unfavorable in terms of mechanical strength and thermal stability. Furthermore, there is relatively slight variability in the X direction because of the limited deflectability of the inner diaphragm. Finally, the tilting axis is overdefined by the inner bearing and the ball guide bushing.